But, researchers have now claimed the condition can be detected in infants at six months by a simple test based on their head and neck control.
The test involves a "pull-to-sit" task, in which the infant is expected to maintain head alignment when being carefully yet firmly pulled by the arms from lying flat on his or her back to sitting up.
It can reveal a lack of postural control in infants that is normally achieved by four months old.
Infants without postural control exhibit "head lag", the researchers said.
Other studies have found that head lag indicates developmental delays in preterm infants and children with cerebral palsy.
But it hadn't been studied in relation to autism.
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"Research aimed at improving early detection of autism has largely focused on measurement of social and communication development," said Dr Rebecca Landa, director of the Centre for Autism and Related Disorders at Kennedy Krieger Institute in Baltimore.
"However, disruption in early motor development may also provide important clues about developmental disorders such as autism," Dr Landa was quoted as saying by LiveScience.
The researchers suggested adding the early test of motor skills to existing behavioural screenings for the risk of autism spectrum disorder (ASD) and also, more broadly, language or social-developmental delays.
The study involved 40 infants at high risk for ASD because a sibling had the disorder.
They were tested at 6, 14, 24 months, then checked at 30 to 36 months for autism.
About 90 per cent of infants diagnosed with ASD at that point had exhibited head lag as infants.
"Our findings show that the evaluation of motor skills should be incorporated with other behavioural assessments to yield insights into the very earliest signs of autism," Landa said.
Researchers from Harvard Medical School and Massachusetts General Hospital have also contributed to the study which was presented at the International Meeting for Autism Research in Toronto.